The basic operation and structure of trunking communication systems and cellular telephone communication systems (cellular communication systems) are known. Trunking communication systems typically comprise a communication channel controller, a limited number of repeaters that transceive information via communication channels, and a plurality of communication units which may be mobile vehicle radios and/or portable radios. Of the communication channels, one is typically chosen to be a control channel. The control channel typically transceives operational information between the communication channel controller and the plurality of communication units such that, for example, the plurality of communication units can access the communication channels. Typically, the trunking communication system has a relatively large geographic coverage area. Depending on the environment that the trunking communication system is located, the coverage area may be approximately 35 miles in radius or greater.
A cellular communication system typically comprises a mobile telephone switching office (MTSO), a plurality of cells, a limited number of communication channels, and a plurality of communication units which may be cellular telephones. Each of the plurality of cells comprises some of the limited number of communication channels, wherein one of the communication channels is designated as the control channel for that cell. The control channel transceives operational information between the plurality of communication units within the cell and the MTSO such that the communication units can place telephone calls via a phone system such as the Public Switched Telephone Network (PSTN). Typically, the coverage area of each cell is relatively small in comparison with that of a typical trunking system. For example, a typical cell coverage area may vary from 2 miles to 10 miles in radius. Because an individual cell coverage area is relatively small, the limited number of communication channels may be reused, at least once, within a geographic region of approximately the same size as a trunking communication system.
Trunking communication systems efficiently support group dispatch which allows multiple communication units to simultaneously access one communication channel. Trunking communications systems may also support individual telephone calls, however, individual calls are an inefficient use of a communication channel due to the large coverage area. In contrast, cellular communications systems are relatively efficient at providing individual telephone calls for communication units because of various communication channel re-use schemes employed. The communication channel re-use schemes allow multiple calls to simultaneously proceed while using the same communication channel. However, cellular communication systems are relatively inefficient at providing group dispatch due to the small coverage area of the cells.
Due to the different configurations of a trunking communication system and a cellular communication system, a typical communication unit, without two receivers and two transmitters, cannot operate in both communication systems. Having two receivers and transmitters adds considerable expense to manufacturing and purchasing of a communication units, thus, for practical reasons, most communication units have only one receiver and one transmitter. However, recent advances in technology have expanded the bandpass of the receivers and transmitters in communication units such that a communication unit is able to transceive in either system. (Note that presently the FCC has designated for a trunking communication system, the frequency range of 806 to 821 Mhz as the transmit region and the frequency range of 850 to 865 Mhz as the receive region, and, for a cellular communication system, the frequency range of 824 through 849 Mhz as the transmit region and 869 to 894 Mhz for the receive region.)
Even with the expanded bandpass of the receiver and transmitter, a typical communication unit is limited to operating in one system at a time due to their technological limitations. Within the receiver, a mixer mixes a received signal with a local oscillator to produce an intermediate signal. The intermediate signal is passed through a bandpass filter which substantially filters out all but one signal at a time which eliminates signals transmitted by the other communication system. The transmitter is limited for similar reasons.
Another limitation that impedes communication units from operating in both systems is that, in many cases, a trunking communication system and a cellular communication system within the same geographic area may be manufactured by different manufacturers. This presents a limitation in that typically one manufacturer does not know, or cannot use, the signalling protocol of the other, which makes accessing that system very difficult.
Therefore, a need exists for a method that will allow a communication unit having one receiver and one transmitter to operate in either a trunking communication system or a cellular communication system, wherein switching from one system to the other will be done transparent to a user of the communication unit regardless of the manufacturers of the systems.